出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2012/08/26 21:47:23」(JST)
Olfactory tubercle | |
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Cresyl Violet Stain of Olfactory Tubercle. Coronal section of the fore brain of a 18 month male mouse (located on most posterior part) | |
Latin | tuberculum olfactorium |
The olfactory tubercle (OT, tuberculum olfactorium) is a multi-sensory processing center in the olfactory cortex that plays a role in reward behaviors. The OT is a composite structure that receives direct input from the olfactory bulb and contains the morphological and histochemical characteristics of the ventral pallidum and the striatum of the forebrain (Heimer & Wilson 1975). In addition, the OT contains tightly packed cell clusters known as the Islands of Calleja, which consist of small granule cells. Even though it’s part of the olfactory cortex and receives direct input from the olfactory bulb, it has not been shown to play a role in processing of odors.
The OT is interconnected with numerous brain regions especially the sensory and arousal/reward centers, thus making it a potentially critical interface between processing of sensory information and the subsequent behavioral responses (Wesson & Wilson 2011). OT has also been shown to play a role in locomotor and attentional behaviors specifically in social and sensory responsiveness (Hitt, Bryon & Modianos 1973) and it may be necessary for behavioral flexibility (Koob et al. 1978).
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The olfactory tubercle was first described by Albert von Kölliker in 1896 who studied them in rats. Since then, there have been several histological and histochemical studies (Koob 1978; Millhouse 1984) done in this area to identify it in other rodents, cats, humans, non-human primates, and other species. Similar studies were done by several authors to find the cell composition and innervations to and from other regions in the OT. Over the years several other methods have been eemployed to find the possible functions and role of the OT in the brain. These began with lesion studies (Gervais 1979; Oades 1981; Asher & Aghajanian 1974; Koob 1978) because the olfactory tuber is too small of an area to record from. Recent developments in technology have made it possible to now place electrodes in this area of the brain so recordings can be done in animals in an awake and vigilant state while they participate in several behavioral studies (Wesson & Wilson 2010; Ikemoto 2002; Ikemoto 2003; Doty 1991)
The olfactory tubercle differs in location and relative size between humans, non-humane primates, rodents, birds and other animals. In most cases, the olfactory tubercle is identified as a round bulge along the basal forebrain anterior to the optic chiasm and posterior to the olfactory peduncle (Wesson & Wilson 2010). In humans and non-human primates, visual identification of the olfactory tubercle is not easy because the basal forebrain bulge is small in these animals (Milhouse & Heimer 1984). With regards to functional anatomy, the olfactory tubercle can be considered to be a part of three larger networks. First it is considered to be part of the basal forebrain, the nucleus accumbens, and the amygdaloid nuclei because of its location along the rostral ventral region of the brain; that is the front bottom part. Second it considered to be part of the olfactory cortex because it receives direct input from the olfactory bulb. Third, it is also considered to be part of the ventral striatum based on anatomy, neurochemical and embryology data.
One of the most striking features of the olfactory tubercle are the closely packed crescent shaped cell clusters which mostly located in layer III and sometimes in layer II. These cells clusters, called the islands of calleja, are innervated by dopaminergic projections from the nucleus accumbens and the substantia nigra suggesting the role that the olfactory tubercle plays in the reward system.
The olfactory tubercle is a multi-sensory processing center due to the number of innervations going to and from other brain regions such as the amygdala, thalamus, hypothalamus, hippocampus, brain stem, auditory and visual sensory fibers, and a number of structures in the reward–arousal system as well as the olfactory cortex. Due to its many innervations from other brain regions, the olfactory tubercle is involved in merging information across the senses, such as olfactory—audition and olfactory—visual integrations, possibly in a behaviorally relevant manner. Thus damage to the olfactory tubercle is likely to affect the functionality of all these area of the brain. Examples of such disruption include changes in normal odor guided behavior, and impairments in modulating state and motivational behavior (Wesson & Wilson 2011) which are common in psychiatric disorders such as schizophrenia (Rupp et al. 2005), dementia (Murphy, Nordin & Jinich 1999) and depression (Negoias et al. 2010).
The olfactory tubercle has been shown to play a large role in behavior. Unilateral lesions in the olfactory tubercle have been shown to alter attention, social and sensory responsiveness and even locomotor behavior (Koob 1978). Bilateral lesions have been shown to reduce copulatory behavior in male rats. The olfactory tubercle has also been shown to be especially involved in reward and addictive behaviors. Rats have been shown to administer cocaine into the olfactory tubercle more than the nucleus accumbens and ventral pallidum, other reward centers in the brain (Ikemoto 2003). In fact they will administer cocaine into the olfactory tubercle at about 200 times per hour and even till death.
Functional contributions of the olfactory tubercle to olfaction are currently unclear; however there is evidence of a perceptual role that it may play. Work from Zelano et al. (2007) suggest that the olfactory tubercle may be crucial in sorting out the sources of olfactory information. This suggests that it may also play a role in odor guided behavior. Thus it may link perception of odor with action through its connections with attention, reward and motivation systems of the basal forebrain (Wesson & Wilson 2011). Functional imaging data from this same group also shows that the olfactory tubercle is highly activated during tasks that engage attention, thus playing a large role in arousal related systems.
As mentioned earlier, because the olfactory tubercle is a component of the ventral striatum, it is heavily interconnected with several affective, reward, and motivation related centers of the brain. It also sits at the interface between the olfactory sensory input and state dependent behavioral modulatory circuits, that is the area that modulates behavior during certain physiological and mental states. Thus the olfactory tubercle may also play an important role in the mediation of odor approach and odor avoidance behavior, probably in a state dependent manner (Gervais 1979).
The olfactory tubercle is generally located at the basal forebrain of the animal within the medial temporal lobe. Specifically parts of the tubercle are included in the olfactory cortex and nested between the optic chiasm and olfactory tract and ventral to the nucleus accumbens. The olfactory tubercle consists of three layers, a molecular layer (layer I), the dense cell layer (layer II) and the multiform layer (layer III) (Millhouse & Heimer 1984). Other than the islands of calleja which are characteristic of the tubercle (Calleja 1893), it is also noted for the being innervated by dopaminergic neurons from the ventral tegmental area. The olfactory tubercle also consist of heterogeneous elements such as medial forebrain bundle and has a ventral extension of the striatal complex. During the 1970s the tubercle was found to contain a striatal component which is filled GABAergic medium spiny neurons. The GABAergic project to the ventral pallidum and receive glutamatergic inputs from cortical regions and dopaminergic inputs from the ventral tegmental area (Millhouse 1987; Meyer 1989).
The ventral portion of the olfactory tubercle consists of three layers whereas the dorsal portion contains dense cell clusters and adjoins the ventral pallidum (within the basal ganglia). The structure of the most ventral and anterior parts of the tubercle can be defined as anatomically defined hills (consisting of gyri and sulci) and clusters of cells.
The most common cell types in the olfactory tubercle are medium sized dense spine cells found predominantly in layer II (dense cell layer). The dendrites of these cells are covered by substance p immunoreactive (S.P.I) axons up into layer III (multiform layer) (Millhouse & Heimer 1984). These cells also project into the nucleus accumbens and caudate putamen thus linking the olfactory tubercle with the pallidum (Fallon 1983). Other medium sized cells reside in layers II and III of the olfactory tubercle as well. These include the spine poor neurons and spindle cells and they differ from the medium sized dense spine cells because they have sparse dendritic trees. The largest cells and most striking feature of the olfactory tubercle are densely packed crescent shaped cell clusters, Islands of Calleja that reside mostly in the dorsal portion of the olfactory tubercle, layer III and can also be found in layer II. The olfactory tubercle also contains three classes of small cell found mostly in layers I and II. The first are pial cells (named as such because of location near pial surface) which look like miniature medium sized dense spine cells. The second are radiate cells and are easily identified by numerous multi directional spineless dendrites. The third, small spine cells, are similar to the pial cells in that they also look like medium sized spine cells except they are not located near the pial surface (Ribak & Fallon 1982).
Migrating cells from several developmental sites come together to form the olfactory tubercle. This includes the ventral ganglionic eminence (found in ventral part of telencephalon where they form bulges in the ventricles that later become the basal ganglia, present only in embryonic stages) and the rostromedial telencephalic wall (of the forebrain) (Garcia-Moreno et al., 2008). Olfactory tubercle neurons originate as early as embryonic day 13 (E13) and the cell development occurs in a layer specific manner. The emergence of the three main layers of the olfactory tubercle begins almost simultaneously. The large neurons in layer III originate from E13 to E16, while the small and medium originate between E15 and E20. Like the small and medium cells in layer III, the cells of layer II and the striatal bridges also originate between E15 and E20 and develop in a lateral to medial gradient (Bayer 1985). The granule cells of the islands of calleja originate between E19 and E22 and continue to migrate into the islands until long after birth (Bedard et al. 2002; De Marchis et al., 2004).
Fibers from the lateral olfactory tract begin branching into the olfactory tubercle around E17. The lateral portion of the olfactory tubercle (which adjoins the olfactory tract) receives the densest fiber input and the medial portion receives light fiber projections (Schowb & Price 1984). This branching continues until completion about the end of the first week after birth.
The olfactory tubercle plays a functional role in the integration of olfactory information with extra modal senses. Auditory sensory information may arrive at the olfactory tubercle via networks involving the hippocampus, ventral pallidum or directly from the olfactory cortex, thus showing a possible role of the olfactory tubercle in olfactory auditory sensory integration (Deadwyler, Foster & Hampson 1987). This convergence has been shown to cause the perception of smoud, caused by the interaction between smell and sound. This possibility has been supported by work from Wesson & Wilson (2010) where olfactory tubercle displayed olfactory–auditory convergence.
Retinal projections have also been found in layer II of the olfactory tubercle suggesting that it constitutes a region of olfactory and visual convergence (Mick, Cooper & Magnin 1993). These visual sensory fibers arrive from the retinal ganglion cells. Thus, the olfactory tubercle may play a role in the perception of odors when a visual source is identified.
As far as olfaction is concerned, in vitro data from some studies suggest that the olfactory tubercle units have the functional capability of other olfactory center neurons in processing odor. It has been suggest that the olfactory tubercle may be crucial in determining the source of olfactory information and respond to odor inhalations which are attended to (Zelano et al. 2007).
The olfactory tubercle has been shown to be primarily concerned with the reception of sensory impulses from olfactory receptors (Adey 1959). Because of its connections to regions like the amygdala and hippocampus, the olfactory tubercle may play a role in behavior. Rats rely heavily on olfactory sensory input from olfactory receptors for behavioral attitudes (Barnet 1963). Studies show that bilateral lesions in the olfactory tubercle significantly reduce stereotyped behavior (McKenzie 1972; Asher & Aghajanian 1974) such as copulatory behavior in male rats and a reduction in sniffing and chewing behaviors (Koob et al. 1978). These stereotyped inhibitions may have been caused by the removal of central neuronal processes other than the dopaminergic cells in the olfactory tubercle. Unilateral lesions have been shown to alter attention, social and sensory responsiveness and even locomotor behavior in rats (Koob et al. 1978).
The dopaminergic neurons from the ventral tegmental area that innervate the olfactory tubercle enable the tubercle to play a significant role in reward and arousal and thus appears to mediate the primary reinforcement of cocaine and generate arousal (Ikemoto 2003). The anteromedial portions of the tubercle have been shown to mediate the most robust rewarding effects of drugs like cocaine and amphetamine. This has been shown in studies where rats learn to self-administer cocaine at significantly high rates into the tubercle. Interestingly, injections of cocaine into the tubercle induce robust locomotion and rearing behavior in rats (Ikemoto 2002).
The multi-sensory nature of the olfactory tubercle and the many innervations it receives from other brain regions, especially the direct input from the olfactory bulb and innervations from the ventral tegmental area, makes it likely to be involved in several psychiatric disorders in which olfaction and dopamine receptors are affected. Many studies have found reduced olfactory sensitivity in patients with major depressive disorders (MDD) and dementia and schizophrenia. Patients with MDD have been shown to have reduced olfactory bulb and olfactory cortex (Negoias et al. 2010) as compared to normal people. In dementias, especially of the Alzheimer’s type, the olfactory bulb, anterior olfactory nucleus and orbitofrontal cortex, all areas that of the brain that process olfaction, are affected. The deficits observed in dementia include decrease in odor threshold sensitivity (Bacon et al. 1998; Nordin & Murphy 1996), odor identification (Doty et al. 1991) and odor memory (Murphy, Nordin & Jinich 1999). Patients with schizophrenia exhibit deficits in olfactory discrimination that is not seen in patients with other psychiatric disorders not mentioned here. Rupp et al. (2005), found that in patients with schizophrenia olfactory sensitivity and discrimination as well as higher order identification abilities are reduced. As mentioned earlier the olfactory tubercle may be involved in the perception of odors due to the inputs received from the bulb and thus by extension may play a role in these psychiatric disorders.
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リンク元 | 「rhinencephalon」「anterior perforated substance」「olfactory cortex」「olfactory tract」「olfactory pathway」 |
関連記事 | 「tubercle」「olfactory」 |
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